1. Field of the Invention
The present invention relates to an ink for ink jet recording comprising as an additive or print density improver a compound which has a specific structure to exert an excellent effect of improving print density or like properties. More particularly, the present invention relates to the print density improver for ink for ink jet recording and a method for improving the print density in ink jet recording involving the use of the print density improver.
2. Description of the Related Art
Ink jet recording method has been rapidly spread and further developed because it requires a reduced material cost, allows a high speed recording, causes little noise during recording and allows easy color recording.
Examples of ink jet recording method include a continuous method involving continuous ejection of droplets and an on-demand method involving ejection of droplets according to an image data signal. Examples of ejection method include a method which allows a piezoelectric element to give pressure that causes a droplet to be ejected, a method which comprises heating the ink to generate bubbles causing a droplet to be ejected, a method involving the use of ultrasonic wave, and a method which uses electrostatic force to suck and discharge a droplet. As inks for these ink jet recording methods there are used aqueous inks, oil-based inks and solid (melt type) inks.
The dyes to be incorporated in these inks for ink jet recording are required to exhibit a high solubility in solvents, allow a high density recording and have a good hue and an excellent fastness to light, heat, active gas in the atmosphere (e.g., oxidizing gas such as NOx and ozone, SOx), water and chemical, a good fixability to image-receiving materials, difficulty in bleeding, a high print density on image-receiving materials, an excellent preservability, no toxicity and a high purity and be available at a low cost. However, it is extremely difficult to meet all these requirements only the properties of dyes.
Under these circumstances, the inventors considered to try to meet the aforementioned demands on ink for ink jet recording by incorporating compounds other than dye in the ink so that the insufficiency in the properties of the dye can be made up for. In some detail, the inventors considered to try to improve print density, light-fastness and ozone resistance by incorporating specific compounds in the ink. In particular, in the case where association is utilized to raise the fastness of dye, the resulting print tends to have a lowered density. In this case, the print density improver of the invention is effective. JP-A-4-339876 discloses that the combined use of an organic amine salt of dicarboxylic acid and a dicarboxylic acid makes it possible to raise the print density on a high quality paper. However, when a dicarboxylic acid is additionally used, the resulting ink has a lowered pH value (acidic) and thus becomes impractical. Further, the dicarboxylic acids (e.g., maleic acid, malic acid) disclosed in JP-A-4-339876 above exert a low effect of enhancing print density according to the inventors' evaluation. JP-A-2003-147245 discloses that the use of an ink composition for ink jet recording comprising an organic phosphonic acid makes it possible to prevent head clogging but makes no reference to print density and fastness in the examples. Japanese Patent No. 3487010 discloses an ink for ink jet recording comprising a compound having a heterocyclic ring and a carboxyl group incorporated therein, but the compound disclosed has only one carboxylic acid group and thus can exert a low effect according to the inventors' evaluation. JP-A-2004-123904 discloses an aqueous ink comprising various chelating agents, but these chelating agents have an effect only on calcium and magnesium. JP-A-2002-317136 and JP-A-2004-59755 disclose an ink for ink jet recording comprising a specific organic phosphonic acid, but this organic phosphonic acid has an effect of preventing scorching. These references make no reference to enhancement of print density.